fix: BigQueryEngineAdapter.get_table_schema() mypy error (#5153)

Author: newtonapple

sqlmesh/core/engine_adapter/bigquery.py

@@ -296,7 +296,9 @@ def get_bq_schema(self, table_name: TableName) -> t.List[bigquery.SchemaField]:
         table = exp.to_table(table_name)
         if len(table.parts) == 3 and "." in table.name:
             self.execute(exp.select("*").from_(table).limit(0))
-            return self._query_job._query_results.schema
+            query_job = self._query_job
+            assert query_job is not None
+            return query_job._query_results.schema
         return self._get_table(table).schema
 
     def columns(